This invention relates to the processing of microcircuits in general and more particularly to an automatic mask alignment system for use in apparatus in which an image on a mask is projected on to a substrate coated with a resist in order to expose the resist for further processing of the substrate.
In the making of micro-circuits the general process followed is that of generating an oxide film on the semiconductor substrate; coating the oxide film with a photoresist and then illuminating the photoresist through a mask to expose selected portions of the resist. After exposure, the photoresist is developed creating a pattern corresponding to either the exposed or unexposed portions depending on the type of photoresist. The remaining photoresist forms a protective cover for the oxide layer which is then etched, for example, in hydrofluoric acid, to expose the layer below the oxide film, whereupon impurities can be infused into the substrate for doping purposes to form transistors and the like. Once this is done, the process may be repeated numerous times with additional oxide films formed, more resist deposited, the resist exposed, developed, and the oxide film etched with further steps of doping or depositing carried out.
The exposure of the photoresist is carried out by using masks which are prepared for that purpose. Separate masks are used for each of the successive steps. If everything is to appear in the proper place on the micro-circuit a high degree of alignment is necessary between steps.
Typically, the substrate which is exposed, is in the form of a wafer which will contain thereon a plurality of identical chips or micro-circuits. The mask, similarly will contain numerous identical patterns, one for each of the chips. The mask is made by taking the pattern to be imaged on each one of the micro-circuits and forming it numerous times following a step and repeat process. Thus, the same pattern is found across the rows and columns of the mask.
One system which is used in carrying out exposure of wafers in this manner is disclosed in detail in U.S. Pat. Nos. 3,975,364; 4,011,011; 4,006,645; for example. The system disclosed therein is one which utilizes manual alignment, with the operator viewing the mask and wafer through an optical system. The system includes a fine alignment system and coarse alignment system along with the projection system. The actual element for alignment is carried out by the device disclosed in U.S. Pat. No. 4,006,645.
Proper alignment requires the ability to align at least two targets, one on each side of a wafer. This is accomplished with the viewing system of the type illustrated on FIG. 4 of U.S. Pat. No. 4,011,011 in which the viewing field contains two images derived from the wafer and mask. This then requires that alignment targets be printed on the mask for this purpose.
The first mask imaged on the wafer does not require alignment since there is no previous pattern on which to align. For subsequent automatic alignment, it is necessary to print targets on at least two sites on the water. After processing is carried out, these targets are present and then are used in the next processing step to accurately align the wafer with the next masks. When alignment is done manually, the nature of the targets is not particularly critical; however, if this alignment is to be carried out automatically, certain problems arise. In automatic alignment, it has generally been the practice to make large targets, i.e., targets which take up the space of two or more micro-circuits. This requires that during the step-and-repeat process of making the mask, the microcircuit pattern be interrupted in at least two places so as to include the large alignment target patterns. Since the relative position of the circuit and alignment patterns can change from one mask to another, alignment of the alignment patterns can lead to misalignment of the circuit patterns. Furthermore, at least two spaces which would normally yield microcircuits are lost to the target.
Thus the need for an improved automatic alignment system which avoids these difficulties becomes evident.